U.S. investigators say the most likely cause of the space shuttle Columbia's disintegration in February was a chunk of hard foam that hit and apparently cracked the left wing shortly after launch. The investigating panel is expected to conclude this in its final report due in one month.

The board's report will blame hard foam that peeled off the orbiter's external fuel tank and struck the left wing 82 seconds after liftoff.

Board member Roger Tetrault says evidence from launch video and tons of shuttle debris indicates that the high speed foam impact is the most likely cause of an opening in the wing's front edge that let scorching gases in and melt the wing after Columbia reentered the atmosphere.

"So when you put all of these things together, it's a pretty compelling story that, in fact, the foam is the most probable cause of the shuttle accident," he said.

Tests on a full scale fiberglass model of the shuttle wing's front edge have shown that the foam, when hurled at high speed, can cause cracks. Further tests will take place in July on the reinforced carbon material actually used to protect the wing's leading edge - material that is not as flexible and durable as fiberglass.

But the chairman of the investigating board, retired admiral Harold Gehman, says its members are not unanimous in their degree of certainty about the foam. This is because the evidence linking it to the wing crack is circumstantial. All pictures of the foam strike are too distant to prove that it caused destruction, and the orbiter broke apart before technicians could see for themselves.

"Do we want to say we think it did, we're sure it did, it might have, we think most likely it did, the board is confident that...? I have 13 different opinions on that and at sometime I'm going to have to lock everybody in a room and come out with one set of words," he explained.

Nevertheless, Admiral Gehman says the board's report will recommend that the U.S. space agency NASA take steps to eliminate or minimize fuel tank foam shedding and to enhance the shuttle's ability to endure a debris strike, whether foam or some other source.

He says the panel will also recommend that NASA devise a way for a shuttle crew to survive an orbital disaster. Columbia's break-up killed all seven astronauts aboard because they had no means of escape.

"It's not just the foam that NASA has to do something about," said Admiral Gehman.

The board has already suggested that NASA use every photographic means available, including U.S. spy satellites, to observe a shuttle during a mission to keep flight controllers informed of its physical condition. That suggestion arose from the finding that NASA officials had rejected requests by engineers to use such assets to see if the foam strike had done any damage.

Admiral Gehman says that nothing in the board's recommendations should prevent NASA from meeting its goal of returning space shuttles to flight by late this year or early next.